The Cell Membrane
AP Biology
2010-2011
Overview
 Cell membrane separates living cell from
nonliving surroundings

thin barrier = 8nm thick
 Controls traffic in & out of the cell


selectively permeable
allows some substances to cross more easily
than others
 hydrophobic vs hydrophilic
 Made of phospholipids, proteins & other
macromolecules
Gr.12 Biology
Phosphate
Phospholipids
 Fatty acid tails

hydrophobic
 Phosphate group head

hydrophilic
Fatty acid
 Arranged as a bilayer
Aaaah,
one of those
structure–function
examples
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Phospholipid bilayer
polar
hydrophilic
heads
nonpolar
hydrophobic
tails
polar
hydrophilic
heads
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More than lipids…
 In 1972, S.J. Singer & G. Nicolson
proposed that membrane proteins are
inserted into the phospholipid bilayer
It’s like a fluid…
It’s like a mosaic…
It’s the
Fluid Mosaic Model!
Gr.12 Biology
The Fluid Mosaic Model
“A sea of phospholipids with floating
icebergs of protein”
 Membrane composed of different
components
 Asymmetrical b/c of different proteins on
either side of membrane
 Lateral protein movement (Frye & Edidin
Heterocaryon Expt)
http://course1.winona.edu/sberg/ANIMTNS/frey.htm
proteins do NOT flip-flop
 membrane is always changing its look

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The Fluid Mosaic Model
 Freeze-Fracture Technique
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Membrane is a collage of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
Glycoprotein
Extracellular fluid
Glycolipid
Phospholipids
Cholesterol
Transmembrane
proteins
Peripheral
protein
Cytoplasm
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Filaments of
cytoskeleton
Membrane fat composition varies
 Fat composition affects flexibility

membrane must be fluid & flexible
 about as fluid as thick salad oil

% unsaturated fatty acids in phospholipids
 keep membrane less viscous
 cold-adapted organisms, like winter wheat
 increase % in autumn

cholesterol in membrane
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Membrane Proteins
 Proteins determine membrane’s specific functions

cell membrane & organelle membranes each have
unique collections of proteins
 Membrane proteins:

peripheral proteins
 loosely bound to surface of membrane
 cell surface identity marker (antigens)

integral proteins
 penetrate lipid bilayer, usually across whole membrane
 transmembrane protein
 transport proteins
 channels, permeases (pumps)
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Why are
proteins the perfect
molecule to build structures
in the cell membrane?
AP Biology
2007-2008
Classes of amino acids
What do these amino acids have in common?
nonpolar & hydrophobic
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Classes of amino acids
What do these amino acids have in common?
I like the
polar ones
the best!
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polar & hydrophilic
Proteins domains anchor molecule
 Within membrane

Polar areas
of protein
nonpolar amino acids
 hydrophobic
 anchors protein
into membrane
 On outer surfaces of
membrane

polar amino acids
 hydrophilic
 extend into
extracellular fluid &
into cytosol
Gr.12 Biology
Nonpolar areas of protein
H+
Examples
Retinal
chromophore
NH2
water channel
in bacteria
Porin monomer
b-pleated sheets
Bacterial
outer
membrane
Nonpolar
(hydrophobic)
a-helices in the
cell membrane
COOH
H+
Cytoplasm
proton pump channel
in photosynthetic bacteria
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function through
conformational change =
shape change
Many Functions of Membrane Proteins
Outside
Plasma
membrane
Inside
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Transporter
Enzyme
activity
Cell surface
receptor
Cell surface
identity marker
Cell adhesion
Attachment to the
cytoskeleton
Membrane carbohydrates
 Play a key role in cell-cell recognition

ability of a cell to distinguish one cell
from another
 antigens
important in organ &
tissue development
 basis for rejection of
foreign cells by
immune system

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Any Questions??
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Movement across the
Cell Membrane
AP Biology
2007-2008
Diffusion
 2nd Law of Thermodynamics
governs biological systems

universe tends towards disorder (entropy)
 Diffusion

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movement from high  low concentration
Diffusion
 Move from HIGH to LOW concentration
“passive transport”
 no energy needed

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diffusion
movement of water
osmosis
Diffusion across cell membrane
 Cell membrane is the boundary between
inside & outside…

separates cell from its environment
Can it be an impenetrable boundary?
NO!
OUT
IN
food
carbohydrates
sugars, proteins
amino acids
lipids
salts, O2, H2O
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OUT
IN
waste
ammonia
salts
CO2
H2O
products
cell needs materials in & products or waste out
Diffusion through phospholipid bilayer
 What molecules can get through directly?
fats & other lipids
 water & small polar molecules

inside cell
lipid
H 2O
salt
 What molecules can
NOT get through
directly?

polar molecules
 glucose, amino acids

NH3 sugar aa
outside cell
 salts

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ions
large molecules
 starches, proteins
Channels through cell membrane
 Membrane becomes semi-permeable
with protein channels

specific channels allow specific material
across cell membrane
inside cell
H+Biology salt
Gr.12
H 2O
aa
sugar
outside cell
Facilitated Diffusion
 Diffusion through protein channels


channels move specific molecules across
cell membrane
facilitated = with help
no energy needed
open channel = fast transport
high
low
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“The Bouncer”
Active Transport
 Cells may need to move molecules against
concentration gradient



shape change transports solute from
one side of membrane to other
protein “pump”
conformational change
“costs” energy = ATP
low
ATP
high
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“The Doorman”
Active transport
 Many models & mechanisms
ATP
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ATP
antiport
symport
Getting through cell membrane
 Passive Transport

Simple diffusion
 diffusion of nonpolar, hydrophobic molecules
 lipids
 high  low concentration gradient

Facilitated transport
 diffusion of polar, hydrophilic molecules
 through a protein channel
 high  low concentration gradient
 Active transport

diffusion against concentration gradient
 low  high


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uses a protein pump
requires ATP
ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
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ATP
How about large molecules?
 Moving large molecules into & out of cell
through vesicles & vacuoles
 endocytosis

 phagocytosis = “cellular eating”
 pinocytosis = “cellular drinking”

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exocytosis
exocytosis
Endocytosis
phagocytosis
fuse with
lysosome for
digestion
pinocytosis
non-specific
process
receptor-mediated
endocytosis
triggered by
molecular
signal
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The Special Case of Water
Movement of water across
the cell membrane
AP Biology
2007-2008
Osmosis is diffusion of water
 Water is very important to life,
so we talk about water separately
 Diffusion of water from
high concentration of water to
low concentration of water

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across a
semi-permeable
membrane
Concentration of water
 Direction of osmosis is determined by
comparing total solute concentrations

Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water
water
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hypotonic
hypertonic
net movement of water
Managing water balance
 Cell survival depends on balancing
water uptake & loss
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freshwater
balanced
saltwater
Managing water balance
 Isotonic

animal cell immersed in
mild salt solution
 example:
blood cells in blood plasma
 problem: none
 no net movement of water

flows across membrane
equally, in both directions
 volume of cell is stable
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balanced
Managing water balance
 Hypotonic

a cell in fresh water
 example: Paramecium
 problem: gains water,
swells & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
ATP
 ATP

plant cells
 turgid
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freshwater
Water regulation
 Contractile vacuole in Paramecium
ATP
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Managing water balance
 Hypertonic

a cell in salt water
 example: shellfish
 problem: lose water & die
 solution: take up water or
pump out salt

plant cells
 plasmolysis = wilt
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saltwater
1991 | 2003
Aquaporins
 Water moves rapidly into & out of cells

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evidence that there were water channels
Peter Agre
Roderick MacKinnon
John Hopkins
Rockefeller
Osmosis…
.05 M
.03 M
Cell (compared to beaker)  hypertonic or hypotonic
Beaker (compared to cell)  hypertonic or hypotonic
Which way does the water flow?  in or out of cell
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Any Questions??
Gr.12 Biology